1. Field of the Invention
The present invention concerns in general terms a method of estimating a channel in a mobile telecommunication system and more particularly a method of downlink channel estimation by a base station.
2. Discussion of the Background
In a mobile telecommunication system, base station transmits signals to mobile terminals and receives signals from the mobile terminals. The transmission channels from the base station to the mobile terminals are called downlink channels and conversely the transmission channels from the mobile terminals to the base station are called uplink channels. Each transmission channel, uplink or downlink, comprises a variety of paths between a transmitter and a receiver, each path being characterised by a delay, a direction of arrival, an attenuation and a phase rotation. The effect of the attenuation and phase rotation can be modelled simply by a complex multiplicative coefficient characteristic of the path.
It is known that an array of antennae can be used for forming beams and/or for cancelling interference in one or more directions. The antenna processing consists of a weighting of the outputs of the different antennae by complex coefficients followed by an adding of the weighted outputs, the coefficients being chosen so as to obtain the required equivalent antenna diagram. It is thus possible to form a beam in the directions of arrival of the useful signal whilst placing zeros in the directions of arrival of the interfering signals. Beam formation (also referred to as channel formation), has been applied to mobile telephony, notably in order to direct a reception beam from a base station to a mobile terminal (up link). The base station is then equipped with an adaptive antenna (referred to as an “intelligent antenna”) capable of pointing in the directions of arrival of the paths of propagation of a signal transmitted by a mobile terminal.
When a mobile telephony system functions according to a code division multiple access (CDMA) mode, the signals coming from the different mobile terminals are separated by filterings adapted to the signatures of the different users. The signatures employed must have good correlation properties, namely low intercorrelation values and a very pronounced autocorrelation peak.
The first of these two characteristics makes it possible to separate the signals coming from the different users. Naturally, in practice, this separation is not perfect and depends on a factor of orthogonality between the different transmission channels.
The second characteristic makes it possible amongst other things to separate in time the correlation peaks output from the adaptive filtering and therefore to isolate the signals of a user being propagated on different paths. In order to improve the signal to noise ratio, the diversity of paths can be exploited by combining these signals in a rake receiver. The complex coefficients used in the different branches of the receiver are chosen so as to be equal to the conjugates of the complex multiplicative coefficients introduced by the paths concerned. To this end, the receiver effects an estimation of the uplink channel. In practice, it determines, by means of pilot symbols transmitted by the mobile terminal, the coefficients of attenuation and the phase rotations undergone by the signal along the paths constituting the channel. The resulting filtering operation is a filtering adapted to the equivalent filter of the channel.
In addition to the uplink channel estimation, it maybe advantageous to have available at the base station an estimation of the downlink channel. This is because the base station can then transmit, in the direction of the uplink paths, signals precompensated for phase so that they are in phase again at the mobile terminal. Such a precompensation has the advantage of improving the signal to noise ratio on reception.
In order to have an estimation of the downlink channel at the base station, it can be envisaged estimating this at the mobile terminal and transmitting the estimate obtained to the base station. However, when the channel exhibits rapid variations in its transfer function, for example when the mobile terminal is moving rapidly, the estimation must be transmitted very frequently, which uses very significant conveying resources. Naturally, conversely, if the estimation is transmitted infrequently, the base station will not be able to follow the change in the downlink channel and will therefore not be able to correctly effect the phase precompensation.